Why did you think it will?
Because each image has its own Alpha channel (The transparent layer). When the images are expanded, there are 5 transparent layers, but when they are flattened, there is only one. At least that's how I'm thinking it works. Assuming that I am right, 1 Alpha layer shouldn't require as much space as 5 Alpha layers.
Or does flattening the images still leave an Alpha layer in between each image? If so, that would explain why the file size didn't decrease.
I posted the question because I am hoping to get a better understanding of how Alpha layers work and I thought somebody here might have an answer.